1. Field of the Invention
The present invention relates to the control of charge and discharge of a secondary battery, and more especially to the control of charge and discharge by which the electric power of charge and discharge is limited on the basis of the environment where a secondary battery is positioned and the state of the secondary battery.
2. Description of the Related Art
An electric vehicle (including a hybrid electric vehicle) obtaining the total or a part of the driving force of the vehicle by an electric motor, has a secondary battery (hereafter, referred to simply as a battery) mounted on the vehicle, and by the electric power accumulated in this battery, said electric motor is driven. Regenerative braking is a characteristic function in such an electric vehicle. In regenerative braking, during vehicle braking, the kinetic energy of the vehicle is transformed into the electric energy by making said electric motor function as an electricity generator. The obtained electric energy is accumulated in the battery and is reused for acceleration or other vehicle needs. Accordingly, with regenerative braking, it is possible to reuse energy is nomally radiated into the atmosphere as thermal energy in a conventional automobile which runs by an internal combustion engine alone, and the efficiency of the energy can considerably be improved.
Here, in order to effectively accumulate the electric power generated during regenerative braking in a battery, it is necessary for the battery to have a corresponding margin of capacity. Furthermore, in a type of hybrid electric vehicle in which the generator is driven by the heat engine mounted on the vehicle and generates the electric power and this electric power can be accumulated in a battery, the electric power accumulated in the battery, that is, the charge level can freely be controlled. Consequently, in such a hybrid electric vehicle, it is desirable that the charge level of a battery be controlled such that the charge level is approximately in the middle state (50.about.60%) between the state of full charge (100%) and the state of no charge (0%), so that the regenerative power may be received, and so that the electric power may be supplied to the electric motor immediately if a request is made.
A battery mounted on an electric vehicle may operate in various service environments. When such a battery is used in a cold area, it is considered that in some cases, the battery is used in an environment at a temperature of -10.degree. C. or less, or sometimes at a temperature of -20.degree. C. or less. Furthermore, use at high temperatures, the raising of battery temperature by operation of the battery, and use in an environment where the temperature 40.degree. C. remains above, must all be considered. When a battery is used in such severe environments, a control corresponding to the characteristics of the battery is necessary. Especially, at a low temperature, there is such a problem that the rate of the chemical reaction in a battery is lowered, and therefore, the voltage is lowered when a large current is allowed to flow, so that a necessary voltage cannot be obtained. Furthermore, at a high temperature, there is such a problem that the degradation of a battery is accelerated. In Japanese Patent Laid-open Publication No. Hei 7-67209, a technique by which the voltage of charge is controlled according to the temperature of the electrolytic solution of the battery, is disclosed.
However, even if the control of the voltage of charge is performed as shown by the technique described in the above publication, that control alone cannot sufficiently restrain the degradation of a battery at a high temperature. Furthermore, in the above publication, it is not recognized that a heavy current should not be allowed to flow at a low temperature. Especially, if the control of the charging voltage is alone performed, the control of the electric power of discharge cannot be performed.
Furthermore, in a case where the charge level of a battery is low, it is necessary to limit the discharge so that the charge level may not become 0%. On the contrary, in a case where the charge level is high, it is necessary to control the charge so that the charge level may not exceed 100%.